test(email): add client compatibility fixture matrix

[TheAggressive]

Summary

• Compile three representative universal-profile scenarios through the production email compiler.
• Check structural expectations for Outlook Word, Gmail, Apple Mail, and the shared universal profile, independently of byte-exact golden fixtures.
• Record accepted limitations and add deliberate markup regressions that prove critical expectations fail when removed.
• Document the evidence, its limits, and the fixture update procedure. Gmail claims are calibrated to its current CSS support documentation.

Verification

• pnpm qa:fast passed (including 19 JS suites, 133 tests).
• pnpm test passed (1,091 PHP tests, 4,995 assertions).
• Focused compatibility suite passed (13 tests, 307 assertions).
• PHPCS on new PHP files, PHP syntax checks, JSON parsing, and git diff --check passed.

Scope

These are structural regression fixtures. They do not execute client sanitizers or prove visual rendering. The navigation output in #102 still needs coverage from this matrix before that PR is merged.

Closes #72.